1. Field of the Invention
The present invention relates to a flat coreless vibration motor suitable for a vibration source of a small mobile communications apparatus or a massaging apparatus.
2. Description of the Related Art
Conventional flat coreless vibration motors are disclosed in Japanese Patent Publication No. 63-290153 and Japanese Patent Laid-open Publication No. 8-10972. As shown in FIG. 13, in both motors, an eccentric rotor 27 is installed in a housing 26 which is formed of a case 24 and a bracket 25. The eccentric rotor 27 is rotatably supported at a rotary shaft 31 through a rotor holder 28 via oilless bearings 29 and 30 respectively arranged at the case 24 and the bracket 25.
When a plurality of armature coils, for example, three armature coils, are disposed to be eccentric at one side of the rotor, it is advantageous that a great amount of movement of the center of gravity can be produced. However, three coils are disadvantageous in view of a cost. Also, in the motor having three armature coils as disclosed in Japanese Utility Model Publication No. 2-33573, the time during which torque is applied by all coils in the course of one rotation is very short and electricity is always applied to two coils to rotate a rotor.
Also, Japanese Patent Publication No. 2000-201467 discloses a two phase flat vibration motor in which two air-core coils are arranged at an open angle of 135xc2x0. However, since only a coil acts as a conductive body contributing torque in the motor having two phase coils, torque is weak during driving and rotation. Also, since the air-core coils make an open angle of 135xc2x0, the amount of eccentricity is small. In practice, a weight formed of tungsten alloy needs to be arranged between the two coils.
A rotor having two air-core coils forming an open angle of 120xc2x0 by removing one air-core coil from a rotor having three air-core coils, is disclosed in Japanese Patent Publication No. 7-85636. This rotor is advantageous in view of cost because a coil is removed. However, torque is not generated evenly according to the angle of rotation during one rotation.
Also, as shown in Japanese Patent No. 3039857, one or two air-core coils are arranged at one side of a rotor so that the rotor can be stopped at a position at which an effective conductive body of each air-core coil contributing torque receives the maximum amount of a magnetic field of a magnet, to remove a driving error.
However, in the rotor stop position maintaining unit, since a magnetic material is arranged in a main magnetic field of a magnet, stop/start torque is large so that a driving voltage of a rotor is increased. Also, since only a part of a main magnetic flux is used, loss of some main magnetic flux occurs. Further, since a magnetic material is arranged between the effective conductive body of a coil and a magnet applying a magnetic force to the effective conductive body of the coil, a gap between the magnet and the coil is widened accordingly so that loss of the main magnetic flux becomes great. In order not to sacrifice the gap, a concave portion is formed by applying pressure to part of an air-core coil and a magnetic material is loaded in the concave portion. In this case, however, the air-core coil may be damaged.
To solve the above-described problems, it is an object of the present invention to provide an axial directional gap type eccentric rotor in which the number of air-core armature coils is reduced to save cost, a magnetic material is arranged at a leakage magnetic flux portion, not in a main magnetic flux, so that there is no need to widen the axial directional gap in which the main magnetic flux is formed and accordingly there is no loss of the main magnetic flux, the rotor can be surely stopped and rotated at the position for generating the maximum torque, and the amount of eccentricity can be made great without adding an auxiliary eccentricity increasing unit, and a flat coreless vibration motor in which cost can be saved by reducing the size of a magnet by using the eccentric rotor.
To achieve the above object, there is provided an axial directional gap type eccentric rotor having a stop position maintaining unit comprising a commutator in which at least one air-core armature coil is arranged on one side of a central axis, an arc-shaped protruding portion protruding from the outer circumference of the commutator toward a stator so that at least part thereof faces the outer circumference portion of a donut-shaped magnet at the side of the stator, and an arc-shaped weight having magnetism arranged at at least part of the arch-shaped protruding portion.
Here, since the arc-shaped weight having magnetism receives a leakage magnetic flux around the side of the magnet and is surely stopped at the boundary position between the N and S poles of the magnet, for example, the eccentric rotor is held at the position where the maximum torque is generated during rotation. Thus, since the movement of the rotor is easy and the arc-shaped weight is not arranged in the main magnetic flux, loss of gap is not generated. Due to the arc-shaped protruding portion and the arc-shaped weight installed at the outer circumference of the commutator, the center of gravity is moved to the outside of the rotor so that vibration of the rotor can be made great.
It is preferred in the present invention that the arc-shaped weight is installed at the outer circumference of the air-core armature coil, is located at the outer side of the magnet in a radial direction, spans substantially the same open angle as an open angle of a magnetic pole of the magnet, and is arranged to face the side of the magnet through a gap.
Here, since the center portion of the arc-shaped weight can be positioned at a neutral portion between the N and S poles, the rotor can surely held at the position where the maximum torque is generated and movement becomes easy.
It is preferred in the present invention that the arc-shaped weight is formed of metal having a specific gravity of 7 or more and the surface of the arc-shaped weight is plated with magnetic material, and that the arc-shaped weight is formed of resin having a specific gravity of 7 or more and comprising a magnetic metal.
The rotor can be made eccentric by the rotor stop position maintaining means. When a copper wire or braze wire is plated with magnetic material, torque can be controlled by controlling the thickness of a plated film.
It is preferred in the present invention that the commutator is formed of a printed wiring board having a plurality of segments, formed of printed wiring, divided by slits, a protruding portion having no electric pole is formed lengthwise in the slit by printed wiring and indented portions are formed at both sides of the protruding portion.
By providing a concave portion at both sides of the inside of the slit, the neighboring segments on the commutator are prevented from electrically connecting one another by the brush. Also, metal foreign material produced due to brushing does not fall on the segments and not prevent brushing. The commutator and protruding portion are integrally and easily provided as printed wiring.
There is provided an axial directional gap type vibration motor having a stop position maintaining unit, the motor comprising an eccentric rotor comprising a commutator including at least one air-core armature coil is arranged on one side of a central axis, an arc-shaped protruding portion protruding from the outer circumference of the commutator toward a stator so that at least part thereof faces the side of the outer circumference portion of a donut-shaped magnet at the side of the stator, and an arc-shaped weight having magnetism arranged at at least part of the arc-shaped protruding portion, a pair of brushes supplying electricity to the eccentric rotor, the magnet facing the eccentric rotor through an axial directional gap and driving the eccentric rotor by using a magnetic force, and a housing accommodating the eccentric rotor so that the arc-shaped protruding portion is located at a space provided between the outer circumference of the magnet and the housing, wherein the arc-shaped weight receives leakage magnetic flux of the magnet.
Here, since the effective conductive portion of the air-core armature coil can be held at the portions of the magnet where the maximum magnetic flux is generated, without sacrificing the main magnetic flux, movement is made easy and the size of the magnet can be made compact. Thus, a flat type vibration motor can be obtained at a low cost.